The present invention relates to a stepping motor control apparatus, and especially to a stepping motor control apparatus for driving a control valve in an engine, such as an idling-speed control valve provided in a bypass path connecting the upper and down streams of a throttle valve.
Currently, fluid flow control valves such as a throttle valve, an idling-speed control valve provided in a throttle-bypass path, an exhaust-gas feedback control valve, etc. are provided at various positions in an engine, and the opening/closing of each fluid flow control valve is driven by a stepping motor controlled by an engine control unit. If the stepping motor that drives the fluid flow control valve malfunctions, this malfunction negatively effects the control of the engine.
For example, a problem called the lost-step state occasionally occurs when operating the stepping motor. In a typical stepping motor, since a sensor to detect the step position of the motor is not provided, the motor-angle control (step control) for the stepping motor is performed by a control signal sent from an engine control unit using an open-loop control method in which the motor-step number to be changed is obtained by calculating the difference between the target step position and the previous step position.
However, in the above open-loop control method, sometimes the required motor-step number is not changed due to a malfunction such as the sticking of the stepping motor. This results in the lost-step state, in which the actual step position differs from the step position that the motor should attain. Thus, since the correct step position cannot be attained in the event of the lost-step state, it has become very important to detect or prevent the lost-step state. In Japanese Patent Application Laid-Open Sho. 63-95895, there is a technique in which if a demand signal for moving the motor to the maximum or minimum position continues for a predetermined period, the actual step position coincides with the obtained and stored step position by moving the motor to the maximum or minimum position.
When the stepping motor or the fluid valve is jammed by a foreign substance which has gotten near the stepping motor rotor or a fluid flow control valve plug, or the rotor or the valve plug is directly stuck by viscous material which has entered the motor or the valve, or by the viscous material freezing, recovery from the lost-step state is difficult until the rotor can begin to rotate, even if the step position is corrected. In the above state, even if driving the motor with the usual high pulse-rate is attempted, due to the time required for current to flow in the motor, it is difficult to flow a sufficient amount of current in the motor. Accordingly, the motor cannot generate a large torque, which makes recovery from the lost-step state very difficult.
If the sticking or freezing occurs in an idling-speed control valve (ISC/V) driven by a stepping motor, since the actual step position of the stepping motor does not change, it results in the lost-step state. When an engine is warmed up, the rotational speed of the engine is gradually increased and exceeds the target rotational speed. If a rotational speed-feedback control operation is performed, the control value of the rotational speed is reduced to the minimum, and the stepping motor can no longer be controlled. Also, although the actual step position does not change, the estimated step position approaches and finally reaches the target step position. At that time, control achieved, and the stepping motor no longer moves. Even if the motor and the valve are released from a stuck or frozen state, the stepping motor is not moved to the target step position. Accordingly, a high-idle state or an unstable idling state continues.